Several strains of Sindbis virus (SV) have been developed in our laboratory and other laboratories that vary in their neurovirulence for weanling mice. The wild type AR-339 and the HR (heat resistant) strains are avirulent by intracerebral inoculation while NSV, HR-SMB and B-BHK are virulent. During the previous granting period we developed and characterized a panel of monoclonal antibodies to SV. A number of these antibodies can distinguish the virulent NSV strain from the avirulent AR-339 strain. We intend to use these reagents to identify the basis for neurovirulence of SV, the mechanism by which antibody can protect from established fatal neurological disease and the mechanism of alphavirus neutralization. To accomplish these goals we will: (1) Determine the biological correlates of neurovirulence by comparing the ability of virulent and avirulent strains of SV to replicate in vivo and in vitro and to cause fusion. (2) Determine the antigenic correlates of neurovirulence by comparatively mapping the epitopes of virulent and avirulent strains of SV and selecting avirulent mutants with monoclonal antibodies that preferentially neutralize virulent virus. (3) Determine the genetic correlates of neurovirulence by T1 RNase fingerprinting and sequencing regions of the genome which are distinct. (4) Determine the mechanism of antibody protection from fatal encephalitis by characterizing the biologic properties of protective monoclonal antibodies, assessing the requirement for Fc, characterizing a blocking factor present in early hyperimmune serum and measureing the entry of antibody into the central nervous system. (5) Determine the mechanism of neutralization of SV using monoclonal antibodies to each of the neurtalizing epitopes identified on the E1 and E2 glycoproteins.